The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties theron or therefore.
(1) Field of the Invention
The present invention relates generally to communication systems, and more particularly to an adaptive modem for communication systems that utilizes multiple modulation and signal processing schemes to minimize detection, demodulation, and decoding errors while maximizing transmission data rates.
(2) Description of the Prior Art
Doppler and multipath are two well-known effects presented by communication channels. The doppler effect is characterized by an alteration of the transmitted signal frequency when there is relative movement between the transmitter and receiver. Alternately, multipath describes the multiple and time-delayed received signals resulting from a single transmission that travels multiple and varying length paths between the transmitter and receiver. U.S. Pat. No. 5,301,167 to Proakis et al. describes the ocean acoustic channel behavior and the resulting variant multipath that must be recognized as a basic channel characteristic. Proakis et al. describe an underwater acoustic communications apparatus using doppler removal, a specialized sample timing control technique, and decision feedback equalization to achieve high data rates for phase coherent modulation and demodulation. Proakis et al. present an apparatus in which digitized data streams are preceded by a synchronization signal and training sequence that are Phase-Shift-Keyed (PSK) modulated. This technique, although more efficient than other Frequency-Shift-Keying (FSK) techniques, provides a single transmit modulation and receive demodulation scheme regardless of channel characteristics. Experimentation with the FSK technique indicates communication difficulties even with a relatively high signal-to-noise ratio (SNR).
There is currently no apparatus that provides multiple modulation schemes to adapt the communication system to changing channel characteristics.
What is needed is an apparatus that provides multiple modulation schemes to provide communication system flexibility for changing communication channel characteristics.
It is a general purpose and object of the present invention to improve communication system error and transmission data rates. It is another object of the present invention to provide such improvement through an apparatus that contains multiple modulation schemes comprising modulation technique and encoder pairings, and corresponding multiple demodulation schemes comprising demodulation technique and decoder pairings.
Another object is to provide an apparatus to measure the communication channel""s doppler and multipath characteristics. A further object is to estimate the communication channel""s scattering function from doppler and multipath measurements and modem position estimates. Still another object of the present invention is to utilize such communication channel scattering function estimate, together with channel characteristic data, strategic information, and propagation models, to select the best available modulation scheme for the communication scenario.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
These objects are accomplished with the present invention by an apparatus that uses multiple modulation schemes comprising modulation technique and encoder pairings. Corresponding demodulation technique and decoder combinations provide demodulation schemes that uniquely correspond to the modulation schemes. As communication system performance and objectives change, different modulation schemes may be selected. Modulation schemes may also be selected upon the communication channel scattering function estimate, and the apparatus estimates the channel scattering function from measurements of the channel""s frequency (doppler) and time (multipath) spreading characteristics. An intelligence module measures the communication channel""s doppler and multipath using a probe signal that is transmitted from a first modem to a second modem. Extrinsic data ports allow modem inputs for modem position estimates, channel characteristic data, strategic information, and propagation models to assist in scattering function estimation and modulation scheme selection. A coding module provides a modulation mode identifier to identify the modulation scheme. The second modem uses the modulation mode identifier to select the correct demodulation scheme. The intelligence module controls any probe signal transmission, scattering function estimate updating, and modulation scheme selection. The first and second modems are identically structured to allow bi-directional scattering function estimation.